Vinyl aromatic monomers, such as styrene, are used extensively for the manufacture of plastics. These monomers undergo undesirable thermal and free radical polymerization during storage, shipping, and particularly during processing. Such polymerization can cause fouling of distillation towers and other equipment used for processing the monomers and can render the monomers unfit for use without further treatment. Accordingly, to minimize polymerization, compounds having polymerization inhibiting activity are commonly added to the monomer recovery stream.
A wide variety of compounds are known in the art and have been employed as polymerization inhibitors. However, while some of these compounds can actually inhibit polymerization (hereinafter referred to as "true inhibitors"), others can merely slow down the polymerization process (hereinafter referred to as "retarders").
True inhibitors completely inhibit polymerization for the period of time during which they are present in the styrene stream. The most frequently utilized true inhibitors are stable nitroxide free radical compounds. U.S. Pat. No. 4,670,131, which is representative of the prior art, discloses the use of stable free radicals, including nitroxides, to inhibit the polymerization of olefinic compounds, such as styrene. Nitroxides are generally recognized as the cornerstone of inhibitor programs because of their superior inhibiting capabilities. Other inhibitors, such as alkyl hydroxylamines, are not as effective in styrene systems at the desired levels.
Unfortunately, true inhibitors are consumed during the course of their activity. This means that following complete consumption, polymerization occurs as if the system was never treated. Therefore, in a plant emergency where the flow of styrene antifoulant is lost, a distillation tower treated with a true inhibitor will become an untreated tower in a very short period of time. This is particularly problematic as polymerization can continue and in effect turn the inside of the tower into solid polystyrene.
Retarders, unlike true inhibitors, do not stop polymerization. Rather, retarders slow down the rate of polymer growth. The compounds commercially employed as retarders are dinitrophenols, such as 2,4- and 2,6-dinitrophenol, as well as alkylated homologues such as 2,4-dinitro-o-cresol and 2,4-dinitro-sec-butylphenol.
The advantage of using a retarder like dinitrophenol in a treatment program is that it is not rapidly consumed. This means that unconsumed retarder can generally be recycled in a styrene recovery process. Moreover, the lack of consumption enables the retarder to maintain distillation tower integrity for an extended period of time in the event of a plant emergency.
Therefore, combining a true inhibitor like nitroxide with dinitrophenolic retarder could effectively control polymerization, even during a plant emergency. The true inhibitor would inhibit polymerization while, in an emergency situation, the retarder would slow polymerization until the emergency could be treated. This type of inhibitor program has been disclosed in the prior art. For example, U.S. Pat. No. 5,254,760 teaches the use of a nitroxide in combination with an aromatic nitro compound, such as dinitro-o-cresol (DNOC), to inhibit the polymerization of styrene.
Unfortunately, although dinitrophenols, such as DNOC, are effective retarders, they are extremely toxic. In addition, dinitrophenols have very low solubility, i.e., less than 5%, in both styrene and its precursor ethylbenzene. Companies that use either of these two products typically make up solutions in hot styrene or ethylbenzene to increase solubility. However, the companies are then dealing with a known toxin dissolved in a hot carcinogen. Although solubility problems can be overcome by using products such as dinitro-sec-butylphenol, the alkyl group does not add any activity to the product. Therefore, while solubility in the hydrocarbons is increased, product activity is decreased.
Furthermore, styrene manufacturers have gone to great lengths to remove air from the product recovery section of their plants. Thus, an inhibitor system must work under anaerobic conditions. The term "anaerobic" is used herein to mean substantially free of oxygen. In other words, although styrene manufacturers attempt to operate air-free processes, trace amounts of oxygen may nonetheless be present. Several known retarders, however, require the presence of oxygen to reduce the amount of polymerization which occurs. For example, U.S. Pat. No. 4,466,905 discloses that phenylenediamines and 2,6-dinitro-p-cresol will inhibit polymerization in the distillation column if oxygen is present.
Accordingly, it would be desirable to provide an improved composition and method for the inhibition of polymerization during the anaerobic production of styrene using a combination of a true inhibitor and a retarder. It would also be desirable to employ a stable nitroxide free radical compound as the true inhibitor and a non-toxic compound as the retarder.